This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-141158, filed May 11, 2001; and No. 2002-131266, filed May 7, 2002, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a frequency converter, more particularly to a frequency converter used for radio communications.
2. Description of the Related Art
A direct conversion receiving system suitable for miniaturization due to the reduced number of parts has been put to practical use with the penetration of radio communication terminal such as portable telephone. However, the direct conversion system uses a local oscillation signal (LO signal) of substantially the same frequency as the receiver signal (RF signal) frequency that is received by a radio antenna. For this reason, the direct conversion system includes a problem such as a self-mixture that the leakage LO signal enters in a receiver signal channel and becomes a jamming signal. As one method to solve this is suggested a method to use LO signal of approximately half frequency of the receiver signal. According to this method, approximately half frequency of RF signal frequency is used as LO signal frequency.
A direct conversion receiver comprises an even harmonic mixer using half frequency of RF signal frequency. This direct conversion receiver has property suitable for the direct conversion because this receiver does not indicate theoretically sensitivity to the LO signal even if the LO signal frequency enters in the receiver signal channel. However, the even harmonic mixer includes a problem that when the LO signal amplitude fluctuates by temperature change, the conversion gain has varied with this fluctuation, that is, the gain of the receiver does not indicate a target value.
Further, the gain greatly varies when the LO signal generator which is sensitive to the temperature change is used. In order to solve this problem, the conventional device has employed a method of inputting the LO signal to a limiting circuit to make the amplitude constant, and then inputting the LO signal to an even harmonic mixer. However, this method needs a filter for eliminating the harmonic component because many harmonic components of the LO signal occur in the limiting circuit. Generally it is difficult to integrate this filter on a chip.
Further, it obstructs an advantage of the direct conversion that the number of parts can be reduced to provide the filter between the LO signal generator and the even harmonic mixer. Since the conversion gain of the even harmonic mixer depends upon fluctuation of the LO signal amplitude, it is difficult to combine the even harmonic mixer with the cheap oscillation circuit by which the LO signal amplitude is fluctuated in easy.
It is an object of the present invention to provide a frequency converter that can utilize an advantage of an even harmonic mixer that the sensitivity deterioration due to self-mixture is small, without increase of parts.
According to an aspect of the present invention, there is provided a frequency converter to which an input signal, a local oscillation signal and a reference direct current signal is supplied, the frequency converter comprising: a variable gain amplifier which amplifies the local oscillation signal according to a gain control signal and outputs an amplified local signal; an even harmonic mixer which is supplied with the input signal and the amplified local oscillation signal and outputs an output signal whose frequency is a sum of a first frequency of the input signal and a second frequency of two or more even numbered times a frequency of the amplified local oscillation signal or a difference between the first frequency and the second frequency; an amplitude detector which is supplied with the amplified local oscillation signal and outputs a direct current signal having an amplitude corresponding to an amplitude of the amplified local oscillation signal; and a comparator which compares the direct current signal of the amplitude detector with the reference direct current signal to generate an output signal as the gain control signal.
According to another aspect of the present invention, there is provided an orthogonal demodulator to which an input signal, a first local oscillation signal, a second local oscillation signal, a first reference signal, and a second reference signal, the orthogonal demodulator comprising: a first frequency converter including: a first variable gain amplifier which amplifies the first local oscillation signal according to a first gain control signal, and outputs an amplified first local oscillation signal; a first even harmonic mixer which is supplied with the input signal and the amplified first local oscillation signal and outputs an output signal whose frequency corresponds to a difference between a frequency of the input signal and a frequency of two or more even numbered times a frequency of the amplified first local oscillation signal; a first amplitude detector which is supplied with the amplified first local oscillation signal and outputs a first direct current signal having an amplitude corresponding to an amplitude of the amplified first local oscillation signal; and a first comparator which compares the first reference direct current signal with the first direct current signal to generate an output signal as the first gain control signal; a second frequency converter including: a second variable gain amplifier which amplifies the second local oscillation signal according to a second gain control signal, and outputs an amplified second local oscillation signal; a second even harmonic mixer which is supplied with the input signal and the amplified second local oscillation signal and outputs an output signal whose frequency corresponds to a difference between a frequency of the input signal and a frequency of two or more even numbered times a frequency of the amplified second local oscillation signal; a second amplitude detector which is supplied with the amplified second local oscillation signal and outputs a second direct current signal having an amplitude corresponding to an amplitude of the amplified second local oscillation signal; and a second comparator which compares the second reference direct current signal with the second direct current signal to generate an output signal as the second gain control signal; and a phase shifter which outputs the first local oscillation signal and the second local oscillation signal with a given phase difference therebetween to the first frequency converter and the second frequency converter.